Volcanic Activity Summary: Over the past month, seismic activity at Okmok volcano has continued to decrease and is now at or very near pre-eruption levels. The last confirmed ash emission at Okmok occurred on August 19. The likelihood of a resumption of eruptive activity at Okmok has greatly diminished, therefore, we are reducing the Aviation Color Code to GREEN and Volcanic Alert Level to NORMAL for Okmok.

Hazard Analysis: [General hazards] Unstable, muddy surfaces and slopes of new volcanic debris within the caldera are prone to liquefaction and collapse. In addition, new lakes, ponds, and deep craters with steep, collapsing banks present hazards to anyone visiting the caldera. The interior of the new tephra cone was hot when imaged with an infrared camera in mid-September. Volcanic gas was not measured; however, noxious gases could be present around this new tephra cone. All drainages leading downslope from the rim of the caldera should be considered hazardous as ash and other loose debris may be remobilized suddenly by heavy rains. The Crater Creek drainage on the north-northeast flank of Okmok should be avoided as it may experience sudden flooding events if water impounded within the caldera breaches new tephra dams.

Remarks: Okmok volcano was in vigorous eruption from July 12 through mid-August, 2008. Energetic, intermittent ash emission from several vents within the caldera blanketed much of the northeast portion of Umnak Island with ash and dusted Unalaska/Dutch Harbor with fine ash on several occasions. Rain-induced remobilization of debris from high on the volcano's flanks produced lahars down many drainages, forming new deltas at the coastline. A new, 200-300 m-high (660-980 ft) tephra cone developed inside the caldera at the primary eruption site. Since late August, seismicity at Okmok has generally declined. Occasional thermal anomalies visible in satellite images likely reflect the still-warm vent area, fumaroles, and/or lake within the caldera.

Okmok volcano is a 6-mile-wide caldera that occupies most of the eastern end of Umnak Island, located 75 miles southwest of Unalaska/Dutch Harbor in the eastern Aleutian Islands. Okmok has had several eruptions in historic time typically consisting of ash emissions occasionally to over 30,000 feet ASL but generally much lower; lava flows crossed the caldera floor in 1945 and 1958. Prior to 2008, the volcano last erupted in February 1997 producing lava flows and intermittent ash emissions over the course of several months.

The nearest settlement is Nikolski, population about 35, roughly 45 miles west of the volcano. A ranch caretaker family lives at Fort Glenn on the flank of the volcano about 6 miles east of the caldera rim.

Next Notice: A new VAN will be issued if conditions change significantly or alert levels are modified. While a VAN is in effect, regularly scheduled updates are posted at http://www.avo.alaska.edu

The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.

(1) VOLCANO OBSERVATORY NOTICE FOR AVIATION (VONA)

(2) Issued:

(20081119/1854Z)

(3) Volcano:

Okmok (CAVW# 1101-29-)

(4) Current Color Code:

GREEN

(5) Previous Color Code:

yellow

(6) Source:

Alaska Volcano Observatory

(7) Notice Number:

2008/A33

(8) Volcano Location:

N 53 deg 23 min W 168 deg 9 min

(9) Area:

Aleutians Alaska

(10) Summit Elevation:

3520 ft (1073 m)

(11) Volcanic Activity Summary:

Over the past month, seismic activity at Okmok volcano has continued to decrease and is now at or very near pre-eruption levels. The last confirmed ash emission at Okmok occurred on August 19. The likelihood of a resumption of eruptive activity at Okmok has greatly diminished, therefore, we are reducing the Aviation Color Code to GREEN and Volcanic Alert Level to NORMAL for Okmok.

(12) Volcanic cloud height:

Unknown

(13) Other volcanic cloud information:

Unknown

(14) Remarks:

Okmok volcano was in vigorous eruption from July 12 through mid-August, 2008. Energetic, intermittent ash emission from several vents within the caldera blanketed much of the northeast portion of Umnak Island with ash and dusted Unalaska/Dutch Harbor with fine ash on several occasions. Rain-induced remobilization of debris from high on the volcano's flanks produced lahars down many drainages, forming new deltas at the coastline. A new, 200-300 m-high (660-980 ft) tephra cone developed inside the caldera at the primary eruption site. Since late August, seismicity at Okmok has generally declined. Occasional thermal anomalies visible in satellite images likely reflect the still-warm vent area, fumaroles, and/or lake within the caldera.

Okmok volcano is a 6-mile-wide caldera that occupies most of the eastern end of Umnak Island, located 75 miles southwest of Unalaska/Dutch Harbor in the eastern Aleutian Islands. Okmok has had several eruptions in historic time typically consisting of ash emissions occasionally to over 30,000 feet ASL but generally much lower; lava flows crossed the caldera floor in 1945 and 1958. Prior to 2008, the volcano last erupted in February 1997 producing lava flows and intermittent ash emissions over the course of several months.

The nearest settlement is Nikolski, population about 35, roughly 45 miles west of the volcano. A ranch caretaker family lives at Fort Glenn on the flank of the volcano about 6 miles east of the caldera rim.

A new VONA will be issued if conditions change significantly or alert levels are modified. While a VONA is in effect, regularly scheduled updates are posted at http://www.avo.alaska.edu

VOLCANO ALERT LEVELS

NORMALVolcano is in typical background, noneruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

ADVISORYVolcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

WATCHVolcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway but poses limited hazards.

WARNINGHazardous eruption is imminent, underway, or suspected.

AVIATION COLOR CODES

GREENVolcano is in typical background, noneruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

YELLOWVolcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

ORANGEVolcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway with no or minor volcanic-ash emissions [ash-plume height specified, if possible].

REDEruption is imminent with significant emission of volcanic ash into the atmosphere likely OR eruption is underway or suspected with significant emission of volcanic ash into the atmosphere [ash-plume height specified, if possible].